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  <div class="section" id="module-struct">
<span id="struct-interpret-bytes-as-packed-binary-data"></span><h1><a class="reference internal" href="#module-struct" title="struct: Interpret bytes as packed binary data."><code class="xref py py-mod docutils literal notranslate"><span class="pre">struct</span></code></a> — Interpret bytes as packed binary data<a class="headerlink" href="#module-struct" title="Permalink to this headline">¶</a></h1>
<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.7/Lib/struct.py">Lib/struct.py</a></p>
<hr class="docutils" id="index-0" />
<p>This module performs conversions between Python values and C structs represented
as Python <a class="reference internal" href="stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a> objects.  This can be used in handling binary data
stored in files or from network connections, among other sources.  It uses
<a class="reference internal" href="#struct-format-strings"><span class="std std-ref">Format Strings</span></a> as compact descriptions of the layout of the C
structs and the intended conversion to/from Python values.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">By default, the result of packing a given C struct includes pad bytes in
order to maintain proper alignment for the C types involved; similarly,
alignment is taken into account when unpacking.  This behavior is chosen so
that the bytes of a packed struct correspond exactly to the layout in memory
of the corresponding C struct.  To handle platform-independent data formats
or omit implicit pad bytes, use <code class="docutils literal notranslate"><span class="pre">standard</span></code> size and alignment instead of
<code class="docutils literal notranslate"><span class="pre">native</span></code> size and alignment: see <a class="reference internal" href="#struct-alignment"><span class="std std-ref">Byte Order, Size, and Alignment</span></a> for details.</p>
</div>
<p>Several <a class="reference internal" href="#module-struct" title="struct: Interpret bytes as packed binary data."><code class="xref py py-mod docutils literal notranslate"><span class="pre">struct</span></code></a> functions (and methods of <a class="reference internal" href="#struct.Struct" title="struct.Struct"><code class="xref py py-class docutils literal notranslate"><span class="pre">Struct</span></code></a>) take a <em>buffer</em>
argument.  This refers to objects that implement the <a class="reference internal" href="../c-api/buffer.html#bufferobjects"><span class="std std-ref">Buffer Protocol</span></a> and
provide either a readable or read-writable buffer.  The most common types used
for that purpose are <a class="reference internal" href="stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a> and <a class="reference internal" href="stdtypes.html#bytearray" title="bytearray"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytearray</span></code></a>, but many other types
that can be viewed as an array of bytes implement the buffer protocol, so that
they can be read/filled without additional copying from a <a class="reference internal" href="stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a> object.</p>
<div class="section" id="functions-and-exceptions">
<h2>Functions and Exceptions<a class="headerlink" href="#functions-and-exceptions" title="Permalink to this headline">¶</a></h2>
<p>The module defines the following exception and functions:</p>
<dl class="exception">
<dt id="struct.error">
<em class="property">exception </em><code class="descclassname">struct.</code><code class="descname">error</code><a class="headerlink" href="#struct.error" title="Permalink to this definition">¶</a></dt>
<dd><p>Exception raised on various occasions; argument is a string describing what
is wrong.</p>
</dd></dl>

<dl class="function">
<dt id="struct.pack">
<code class="descclassname">struct.</code><code class="descname">pack</code><span class="sig-paren">(</span><em>format</em>, <em>v1</em>, <em>v2</em>, <em>...</em><span class="sig-paren">)</span><a class="headerlink" href="#struct.pack" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a bytes object containing the values <em>v1</em>, <em>v2</em>, … packed according
to the format string <em>format</em>.  The arguments must match the values required by
the format exactly.</p>
</dd></dl>

<dl class="function">
<dt id="struct.pack_into">
<code class="descclassname">struct.</code><code class="descname">pack_into</code><span class="sig-paren">(</span><em>format</em>, <em>buffer</em>, <em>offset</em>, <em>v1</em>, <em>v2</em>, <em>...</em><span class="sig-paren">)</span><a class="headerlink" href="#struct.pack_into" title="Permalink to this definition">¶</a></dt>
<dd><p>Pack the values <em>v1</em>, <em>v2</em>, … according to the format string <em>format</em> and
write the packed bytes into the writable buffer <em>buffer</em> starting at
position <em>offset</em>.  Note that <em>offset</em> is a required argument.</p>
</dd></dl>

<dl class="function">
<dt id="struct.unpack">
<code class="descclassname">struct.</code><code class="descname">unpack</code><span class="sig-paren">(</span><em>format</em>, <em>buffer</em><span class="sig-paren">)</span><a class="headerlink" href="#struct.unpack" title="Permalink to this definition">¶</a></dt>
<dd><p>Unpack from the buffer <em>buffer</em> (presumably packed by <code class="docutils literal notranslate"><span class="pre">pack(format,</span> <span class="pre">...)</span></code>)
according to the format string <em>format</em>.  The result is a tuple even if it
contains exactly one item.  The buffer’s size in bytes must match the
size required by the format, as reflected by <a class="reference internal" href="#struct.calcsize" title="struct.calcsize"><code class="xref py py-func docutils literal notranslate"><span class="pre">calcsize()</span></code></a>.</p>
</dd></dl>

<dl class="function">
<dt id="struct.unpack_from">
<code class="descclassname">struct.</code><code class="descname">unpack_from</code><span class="sig-paren">(</span><em>format</em>, <em>buffer</em>, <em>offset=0</em><span class="sig-paren">)</span><a class="headerlink" href="#struct.unpack_from" title="Permalink to this definition">¶</a></dt>
<dd><p>Unpack from <em>buffer</em> starting at position <em>offset</em>, according to the format
string <em>format</em>.  The result is a tuple even if it contains exactly one
item.  The buffer’s size in bytes, minus <em>offset</em>, must be at least
the size required by the format, as reflected by <a class="reference internal" href="#struct.calcsize" title="struct.calcsize"><code class="xref py py-func docutils literal notranslate"><span class="pre">calcsize()</span></code></a>.</p>
</dd></dl>

<dl class="function">
<dt id="struct.iter_unpack">
<code class="descclassname">struct.</code><code class="descname">iter_unpack</code><span class="sig-paren">(</span><em>format</em>, <em>buffer</em><span class="sig-paren">)</span><a class="headerlink" href="#struct.iter_unpack" title="Permalink to this definition">¶</a></dt>
<dd><p>Iteratively unpack from the buffer <em>buffer</em> according to the format
string <em>format</em>.  This function returns an iterator which will read
equally-sized chunks from the buffer until all its contents have been
consumed.  The buffer’s size in bytes must be a multiple of the size
required by the format, as reflected by <a class="reference internal" href="#struct.calcsize" title="struct.calcsize"><code class="xref py py-func docutils literal notranslate"><span class="pre">calcsize()</span></code></a>.</p>
<p>Each iteration yields a tuple as specified by the format string.</p>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.4.</span></p>
</div>
</dd></dl>

<dl class="function">
<dt id="struct.calcsize">
<code class="descclassname">struct.</code><code class="descname">calcsize</code><span class="sig-paren">(</span><em>format</em><span class="sig-paren">)</span><a class="headerlink" href="#struct.calcsize" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the size of the struct (and hence of the bytes object produced by
<code class="docutils literal notranslate"><span class="pre">pack(format,</span> <span class="pre">...)</span></code>) corresponding to the format string <em>format</em>.</p>
</dd></dl>

</div>
<div class="section" id="format-strings">
<span id="struct-format-strings"></span><h2>Format Strings<a class="headerlink" href="#format-strings" title="Permalink to this headline">¶</a></h2>
<p>Format strings are the mechanism used to specify the expected layout when
packing and unpacking data.  They are built up from <a class="reference internal" href="#format-characters"><span class="std std-ref">Format Characters</span></a>,
which specify the type of data being packed/unpacked.  In addition, there are
special characters for controlling the <a class="reference internal" href="#struct-alignment"><span class="std std-ref">Byte Order, Size, and Alignment</span></a>.</p>
<div class="section" id="byte-order-size-and-alignment">
<span id="struct-alignment"></span><h3>Byte Order, Size, and Alignment<a class="headerlink" href="#byte-order-size-and-alignment" title="Permalink to this headline">¶</a></h3>
<p>By default, C types are represented in the machine’s native format and byte
order, and properly aligned by skipping pad bytes if necessary (according to the
rules used by the C compiler).</p>
<p id="index-1">Alternatively, the first character of the format string can be used to indicate
the byte order, size and alignment of the packed data, according to the
following table:</p>
<table border="1" class="docutils">
<colgroup>
<col width="20%" />
<col width="43%" />
<col width="18%" />
<col width="20%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Character</th>
<th class="head">Byte order</th>
<th class="head">Size</th>
<th class="head">Alignment</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">&#64;</span></code></td>
<td>native</td>
<td>native</td>
<td>native</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">=</span></code></td>
<td>native</td>
<td>standard</td>
<td>none</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">&lt;</span></code></td>
<td>little-endian</td>
<td>standard</td>
<td>none</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">&gt;</span></code></td>
<td>big-endian</td>
<td>standard</td>
<td>none</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">!</span></code></td>
<td>network (= big-endian)</td>
<td>standard</td>
<td>none</td>
</tr>
</tbody>
</table>
<p>If the first character is not one of these, <code class="docutils literal notranslate"><span class="pre">'&#64;'</span></code> is assumed.</p>
<p>Native byte order is big-endian or little-endian, depending on the host
system. For example, Intel x86 and AMD64 (x86-64) are little-endian;
Motorola 68000 and PowerPC G5 are big-endian; ARM and Intel Itanium feature
switchable endianness (bi-endian). Use <code class="docutils literal notranslate"><span class="pre">sys.byteorder</span></code> to check the
endianness of your system.</p>
<p>Native size and alignment are determined using the C compiler’s
<code class="docutils literal notranslate"><span class="pre">sizeof</span></code> expression.  This is always combined with native byte order.</p>
<p>Standard size depends only on the format character;  see the table in
the <a class="reference internal" href="#format-characters"><span class="std std-ref">Format Characters</span></a> section.</p>
<p>Note the difference between <code class="docutils literal notranslate"><span class="pre">'&#64;'</span></code> and <code class="docutils literal notranslate"><span class="pre">'='</span></code>: both use native byte order, but
the size and alignment of the latter is standardized.</p>
<p>The form <code class="docutils literal notranslate"><span class="pre">'!'</span></code> is available for those poor souls who claim they can’t remember
whether network byte order is big-endian or little-endian.</p>
<p>There is no way to indicate non-native byte order (force byte-swapping); use the
appropriate choice of <code class="docutils literal notranslate"><span class="pre">'&lt;'</span></code> or <code class="docutils literal notranslate"><span class="pre">'&gt;'</span></code>.</p>
<p>Notes:</p>
<ol class="arabic simple">
<li>Padding is only automatically added between successive structure members.
No padding is added at the beginning or the end of the encoded struct.</li>
<li>No padding is added when using non-native size and alignment, e.g.
with ‘&lt;’, ‘&gt;’, ‘=’, and ‘!’.</li>
<li>To align the end of a structure to the alignment requirement of a
particular type, end the format with the code for that type with a repeat
count of zero.  See <a class="reference internal" href="#struct-examples"><span class="std std-ref">Examples</span></a>.</li>
</ol>
</div>
<div class="section" id="format-characters">
<span id="id1"></span><h3>Format Characters<a class="headerlink" href="#format-characters" title="Permalink to this headline">¶</a></h3>
<p>Format characters have the following meaning; the conversion between C and
Python values should be obvious given their types.  The ‘Standard size’ column
refers to the size of the packed value in bytes when using standard size; that
is, when the format string starts with one of <code class="docutils literal notranslate"><span class="pre">'&lt;'</span></code>, <code class="docutils literal notranslate"><span class="pre">'&gt;'</span></code>, <code class="docutils literal notranslate"><span class="pre">'!'</span></code> or
<code class="docutils literal notranslate"><span class="pre">'='</span></code>.  When using native size, the size of the packed value is
platform-dependent.</p>
<table border="1" class="docutils">
<colgroup>
<col width="10%" />
<col width="32%" />
<col width="24%" />
<col width="20%" />
<col width="15%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Format</th>
<th class="head">C Type</th>
<th class="head">Python type</th>
<th class="head">Standard size</th>
<th class="head">Notes</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">x</span></code></td>
<td>pad byte</td>
<td>no value</td>
<td>&#160;</td>
<td>&#160;</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">c</span></code></td>
<td><code class="xref c c-type docutils literal notranslate"><span class="pre">char</span></code></td>
<td>bytes of length 1</td>
<td>1</td>
<td>&#160;</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">b</span></code></td>
<td><code class="xref c c-type docutils literal notranslate"><span class="pre">signed</span> <span class="pre">char</span></code></td>
<td>integer</td>
<td>1</td>
<td>(1),(3)</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">B</span></code></td>
<td><code class="xref c c-type docutils literal notranslate"><span class="pre">unsigned</span> <span class="pre">char</span></code></td>
<td>integer</td>
<td>1</td>
<td>(3)</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">?</span></code></td>
<td><code class="xref c c-type docutils literal notranslate"><span class="pre">_Bool</span></code></td>
<td>bool</td>
<td>1</td>
<td>(1)</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">h</span></code></td>
<td><code class="xref c c-type docutils literal notranslate"><span class="pre">short</span></code></td>
<td>integer</td>
<td>2</td>
<td>(3)</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">H</span></code></td>
<td><code class="xref c c-type docutils literal notranslate"><span class="pre">unsigned</span> <span class="pre">short</span></code></td>
<td>integer</td>
<td>2</td>
<td>(3)</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">i</span></code></td>
<td><code class="xref c c-type docutils literal notranslate"><span class="pre">int</span></code></td>
<td>integer</td>
<td>4</td>
<td>(3)</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">I</span></code></td>
<td><code class="xref c c-type docutils literal notranslate"><span class="pre">unsigned</span> <span class="pre">int</span></code></td>
<td>integer</td>
<td>4</td>
<td>(3)</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">l</span></code></td>
<td><code class="xref c c-type docutils literal notranslate"><span class="pre">long</span></code></td>
<td>integer</td>
<td>4</td>
<td>(3)</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">L</span></code></td>
<td><code class="xref c c-type docutils literal notranslate"><span class="pre">unsigned</span> <span class="pre">long</span></code></td>
<td>integer</td>
<td>4</td>
<td>(3)</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">q</span></code></td>
<td><code class="xref c c-type docutils literal notranslate"><span class="pre">long</span> <span class="pre">long</span></code></td>
<td>integer</td>
<td>8</td>
<td>(2), (3)</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">Q</span></code></td>
<td><code class="xref c c-type docutils literal notranslate"><span class="pre">unsigned</span> <span class="pre">long</span>
<span class="pre">long</span></code></td>
<td>integer</td>
<td>8</td>
<td>(2), (3)</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">n</span></code></td>
<td><code class="xref c c-type docutils literal notranslate"><span class="pre">ssize_t</span></code></td>
<td>integer</td>
<td>&#160;</td>
<td>(4)</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">N</span></code></td>
<td><code class="xref c c-type docutils literal notranslate"><span class="pre">size_t</span></code></td>
<td>integer</td>
<td>&#160;</td>
<td>(4)</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">e</span></code></td>
<td>(7)</td>
<td>float</td>
<td>2</td>
<td>(5)</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">f</span></code></td>
<td><code class="xref c c-type docutils literal notranslate"><span class="pre">float</span></code></td>
<td>float</td>
<td>4</td>
<td>(5)</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">d</span></code></td>
<td><code class="xref c c-type docutils literal notranslate"><span class="pre">double</span></code></td>
<td>float</td>
<td>8</td>
<td>(5)</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">s</span></code></td>
<td><code class="xref c c-type docutils literal notranslate"><span class="pre">char[]</span></code></td>
<td>bytes</td>
<td>&#160;</td>
<td>&#160;</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">p</span></code></td>
<td><code class="xref c c-type docutils literal notranslate"><span class="pre">char[]</span></code></td>
<td>bytes</td>
<td>&#160;</td>
<td>&#160;</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">P</span></code></td>
<td><code class="xref c c-type docutils literal notranslate"><span class="pre">void</span> <span class="pre">*</span></code></td>
<td>integer</td>
<td>&#160;</td>
<td>(6)</td>
</tr>
</tbody>
</table>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.3: </span>Added support for the <code class="docutils literal notranslate"><span class="pre">'n'</span></code> and <code class="docutils literal notranslate"><span class="pre">'N'</span></code> formats.</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.6: </span>Added support for the <code class="docutils literal notranslate"><span class="pre">'e'</span></code> format.</p>
</div>
<p>Notes:</p>
<ol class="arabic">
<li><p id="index-2">The <code class="docutils literal notranslate"><span class="pre">'?'</span></code> conversion code corresponds to the <code class="xref c c-type docutils literal notranslate"><span class="pre">_Bool</span></code> type defined by
C99. If this type is not available, it is simulated using a <code class="xref c c-type docutils literal notranslate"><span class="pre">char</span></code>. In
standard mode, it is always represented by one byte.</p>
</li>
<li><p class="first">The <code class="docutils literal notranslate"><span class="pre">'q'</span></code> and <code class="docutils literal notranslate"><span class="pre">'Q'</span></code> conversion codes are available in native mode only if
the platform C compiler supports C <code class="xref c c-type docutils literal notranslate"><span class="pre">long</span> <span class="pre">long</span></code>, or, on Windows,
<code class="xref c c-type docutils literal notranslate"><span class="pre">__int64</span></code>.  They are always available in standard modes.</p>
</li>
<li><p class="first">When attempting to pack a non-integer using any of the integer conversion
codes, if the non-integer has a <a class="reference internal" href="../reference/datamodel.html#object.__index__" title="object.__index__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__index__()</span></code></a> method then that method is
called to convert the argument to an integer before packing.</p>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.2: </span>Use of the <a class="reference internal" href="../reference/datamodel.html#object.__index__" title="object.__index__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__index__()</span></code></a> method for non-integers is new in 3.2.</p>
</div>
</li>
<li><p class="first">The <code class="docutils literal notranslate"><span class="pre">'n'</span></code> and <code class="docutils literal notranslate"><span class="pre">'N'</span></code> conversion codes are only available for the native
size (selected as the default or with the <code class="docutils literal notranslate"><span class="pre">'&#64;'</span></code> byte order character).
For the standard size, you can use whichever of the other integer formats
fits your application.</p>
</li>
<li><p class="first">For the <code class="docutils literal notranslate"><span class="pre">'f'</span></code>, <code class="docutils literal notranslate"><span class="pre">'d'</span></code> and <code class="docutils literal notranslate"><span class="pre">'e'</span></code> conversion codes, the packed
representation uses the IEEE 754 binary32, binary64 or binary16 format (for
<code class="docutils literal notranslate"><span class="pre">'f'</span></code>, <code class="docutils literal notranslate"><span class="pre">'d'</span></code> or <code class="docutils literal notranslate"><span class="pre">'e'</span></code> respectively), regardless of the floating-point
format used by the platform.</p>
</li>
<li><p class="first">The <code class="docutils literal notranslate"><span class="pre">'P'</span></code> format character is only available for the native byte ordering
(selected as the default or with the <code class="docutils literal notranslate"><span class="pre">'&#64;'</span></code> byte order character). The byte
order character <code class="docutils literal notranslate"><span class="pre">'='</span></code> chooses to use little- or big-endian ordering based
on the host system. The struct module does not interpret this as native
ordering, so the <code class="docutils literal notranslate"><span class="pre">'P'</span></code> format is not available.</p>
</li>
<li><p class="first">The IEEE 754 binary16 “half precision” type was introduced in the 2008
revision of the <a class="reference external" href="https://en.wikipedia.org/wiki/IEEE_floating_point#IEEE_754-2008">IEEE 754 standard</a>. It has a sign
bit, a 5-bit exponent and 11-bit precision (with 10 bits explicitly stored),
and can represent numbers between approximately <code class="docutils literal notranslate"><span class="pre">6.1e-05</span></code> and <code class="docutils literal notranslate"><span class="pre">6.5e+04</span></code>
at full precision. This type is not widely supported by C compilers: on a
typical machine, an unsigned short can be used for storage, but not for math
operations. See the Wikipedia page on the <a class="reference external" href="https://en.wikipedia.org/wiki/Half-precision_floating-point_format">half-precision floating-point
format</a> for more information.</p>
</li>
</ol>
<p>A format character may be preceded by an integral repeat count.  For example,
the format string <code class="docutils literal notranslate"><span class="pre">'4h'</span></code> means exactly the same as <code class="docutils literal notranslate"><span class="pre">'hhhh'</span></code>.</p>
<p>Whitespace characters between formats are ignored; a count and its format must
not contain whitespace though.</p>
<p>For the <code class="docutils literal notranslate"><span class="pre">'s'</span></code> format character, the count is interpreted as the length of the
bytes, not a repeat count like for the other format characters; for example,
<code class="docutils literal notranslate"><span class="pre">'10s'</span></code> means a single 10-byte string, while <code class="docutils literal notranslate"><span class="pre">'10c'</span></code> means 10 characters.
If a count is not given, it defaults to 1.  For packing, the string is
truncated or padded with null bytes as appropriate to make it fit. For
unpacking, the resulting bytes object always has exactly the specified number
of bytes.  As a special case, <code class="docutils literal notranslate"><span class="pre">'0s'</span></code> means a single, empty string (while
<code class="docutils literal notranslate"><span class="pre">'0c'</span></code> means 0 characters).</p>
<p>When packing a value <code class="docutils literal notranslate"><span class="pre">x</span></code> using one of the integer formats (<code class="docutils literal notranslate"><span class="pre">'b'</span></code>,
<code class="docutils literal notranslate"><span class="pre">'B'</span></code>, <code class="docutils literal notranslate"><span class="pre">'h'</span></code>, <code class="docutils literal notranslate"><span class="pre">'H'</span></code>, <code class="docutils literal notranslate"><span class="pre">'i'</span></code>, <code class="docutils literal notranslate"><span class="pre">'I'</span></code>, <code class="docutils literal notranslate"><span class="pre">'l'</span></code>, <code class="docutils literal notranslate"><span class="pre">'L'</span></code>,
<code class="docutils literal notranslate"><span class="pre">'q'</span></code>, <code class="docutils literal notranslate"><span class="pre">'Q'</span></code>), if <code class="docutils literal notranslate"><span class="pre">x</span></code> is outside the valid range for that format
then <a class="reference internal" href="#struct.error" title="struct.error"><code class="xref py py-exc docutils literal notranslate"><span class="pre">struct.error</span></code></a> is raised.</p>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.1: </span>In 3.0, some of the integer formats wrapped out-of-range values and
raised <a class="reference internal" href="exceptions.html#DeprecationWarning" title="DeprecationWarning"><code class="xref py py-exc docutils literal notranslate"><span class="pre">DeprecationWarning</span></code></a> instead of <a class="reference internal" href="#struct.error" title="struct.error"><code class="xref py py-exc docutils literal notranslate"><span class="pre">struct.error</span></code></a>.</p>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">'p'</span></code> format character encodes a “Pascal string”, meaning a short
variable-length string stored in a <em>fixed number of bytes</em>, given by the count.
The first byte stored is the length of the string, or 255, whichever is
smaller.  The bytes of the string follow.  If the string passed in to
<a class="reference internal" href="#struct.pack" title="struct.pack"><code class="xref py py-func docutils literal notranslate"><span class="pre">pack()</span></code></a> is too long (longer than the count minus 1), only the leading
<code class="docutils literal notranslate"><span class="pre">count-1</span></code> bytes of the string are stored.  If the string is shorter than
<code class="docutils literal notranslate"><span class="pre">count-1</span></code>, it is padded with null bytes so that exactly count bytes in all
are used.  Note that for <a class="reference internal" href="#struct.unpack" title="struct.unpack"><code class="xref py py-func docutils literal notranslate"><span class="pre">unpack()</span></code></a>, the <code class="docutils literal notranslate"><span class="pre">'p'</span></code> format character consumes
<code class="docutils literal notranslate"><span class="pre">count</span></code> bytes, but that the string returned can never contain more than 255
bytes.</p>
<p id="index-3">For the <code class="docutils literal notranslate"><span class="pre">'?'</span></code> format character, the return value is either <a class="reference internal" href="constants.html#True" title="True"><code class="xref py py-const docutils literal notranslate"><span class="pre">True</span></code></a> or
<a class="reference internal" href="constants.html#False" title="False"><code class="xref py py-const docutils literal notranslate"><span class="pre">False</span></code></a>. When packing, the truth value of the argument object is used.
Either 0 or 1 in the native or standard bool representation will be packed, and
any non-zero value will be <code class="docutils literal notranslate"><span class="pre">True</span></code> when unpacking.</p>
</div>
<div class="section" id="examples">
<span id="struct-examples"></span><h3>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h3>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">All examples assume a native byte order, size, and alignment with a
big-endian machine.</p>
</div>
<p>A basic example of packing/unpacking three integers:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">struct</span> <span class="k">import</span> <span class="o">*</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">pack</span><span class="p">(</span><span class="s1">&#39;hhl&#39;</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span>
<span class="go">b&#39;\x00\x01\x00\x02\x00\x00\x00\x03&#39;</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">unpack</span><span class="p">(</span><span class="s1">&#39;hhl&#39;</span><span class="p">,</span> <span class="sa">b</span><span class="s1">&#39;</span><span class="se">\x00\x01\x00\x02\x00\x00\x00\x03</span><span class="s1">&#39;</span><span class="p">)</span>
<span class="go">(1, 2, 3)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">calcsize</span><span class="p">(</span><span class="s1">&#39;hhl&#39;</span><span class="p">)</span>
<span class="go">8</span>
</pre></div>
</div>
<p>Unpacked fields can be named by assigning them to variables or by wrapping
the result in a named tuple:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">record</span> <span class="o">=</span> <span class="sa">b</span><span class="s1">&#39;raymond   </span><span class="se">\x32\x12\x08\x01\x08</span><span class="s1">&#39;</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">name</span><span class="p">,</span> <span class="n">serialnum</span><span class="p">,</span> <span class="n">school</span><span class="p">,</span> <span class="n">gradelevel</span> <span class="o">=</span> <span class="n">unpack</span><span class="p">(</span><span class="s1">&#39;&lt;10sHHb&#39;</span><span class="p">,</span> <span class="n">record</span><span class="p">)</span>

<span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">collections</span> <span class="k">import</span> <span class="n">namedtuple</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">Student</span> <span class="o">=</span> <span class="n">namedtuple</span><span class="p">(</span><span class="s1">&#39;Student&#39;</span><span class="p">,</span> <span class="s1">&#39;name serialnum school gradelevel&#39;</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">Student</span><span class="o">.</span><span class="n">_make</span><span class="p">(</span><span class="n">unpack</span><span class="p">(</span><span class="s1">&#39;&lt;10sHHb&#39;</span><span class="p">,</span> <span class="n">record</span><span class="p">))</span>
<span class="go">Student(name=b&#39;raymond   &#39;, serialnum=4658, school=264, gradelevel=8)</span>
</pre></div>
</div>
<p>The ordering of format characters may have an impact on size since the padding
needed to satisfy alignment requirements is different:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">pack</span><span class="p">(</span><span class="s1">&#39;ci&#39;</span><span class="p">,</span> <span class="sa">b</span><span class="s1">&#39;*&#39;</span><span class="p">,</span> <span class="mh">0x12131415</span><span class="p">)</span>
<span class="go">b&#39;*\x00\x00\x00\x12\x13\x14\x15&#39;</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">pack</span><span class="p">(</span><span class="s1">&#39;ic&#39;</span><span class="p">,</span> <span class="mh">0x12131415</span><span class="p">,</span> <span class="sa">b</span><span class="s1">&#39;*&#39;</span><span class="p">)</span>
<span class="go">b&#39;\x12\x13\x14\x15*&#39;</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">calcsize</span><span class="p">(</span><span class="s1">&#39;ci&#39;</span><span class="p">)</span>
<span class="go">8</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">calcsize</span><span class="p">(</span><span class="s1">&#39;ic&#39;</span><span class="p">)</span>
<span class="go">5</span>
</pre></div>
</div>
<p>The following format <code class="docutils literal notranslate"><span class="pre">'llh0l'</span></code> specifies two pad bytes at the end, assuming
longs are aligned on 4-byte boundaries:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">pack</span><span class="p">(</span><span class="s1">&#39;llh0l&#39;</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span>
<span class="go">b&#39;\x00\x00\x00\x01\x00\x00\x00\x02\x00\x03\x00\x00&#39;</span>
</pre></div>
</div>
<p>This only works when native size and alignment are in effect; standard size and
alignment does not enforce any alignment.</p>
<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt>Module <a class="reference internal" href="array.html#module-array" title="array: Space efficient arrays of uniformly typed numeric values."><code class="xref py py-mod docutils literal notranslate"><span class="pre">array</span></code></a></dt>
<dd>Packed binary storage of homogeneous data.</dd>
<dt>Module <a class="reference internal" href="xdrlib.html#module-xdrlib" title="xdrlib: Encoders and decoders for the External Data Representation (XDR)."><code class="xref py py-mod docutils literal notranslate"><span class="pre">xdrlib</span></code></a></dt>
<dd>Packing and unpacking of XDR data.</dd>
</dl>
</div>
</div>
</div>
<div class="section" id="classes">
<span id="struct-objects"></span><h2>Classes<a class="headerlink" href="#classes" title="Permalink to this headline">¶</a></h2>
<p>The <a class="reference internal" href="#module-struct" title="struct: Interpret bytes as packed binary data."><code class="xref py py-mod docutils literal notranslate"><span class="pre">struct</span></code></a> module also defines the following type:</p>
<dl class="class">
<dt id="struct.Struct">
<em class="property">class </em><code class="descclassname">struct.</code><code class="descname">Struct</code><span class="sig-paren">(</span><em>format</em><span class="sig-paren">)</span><a class="headerlink" href="#struct.Struct" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a new Struct object which writes and reads binary data according to
the format string <em>format</em>.  Creating a Struct object once and calling its
methods is more efficient than calling the <a class="reference internal" href="#module-struct" title="struct: Interpret bytes as packed binary data."><code class="xref py py-mod docutils literal notranslate"><span class="pre">struct</span></code></a> functions with the
same format since the format string only needs to be compiled once.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The compiled versions of the most recent format strings passed to
<a class="reference internal" href="#struct.Struct" title="struct.Struct"><code class="xref py py-class docutils literal notranslate"><span class="pre">Struct</span></code></a> and the module-level functions are cached, so programs
that use only a few format strings needn’t worry about reusing a single
<a class="reference internal" href="#struct.Struct" title="struct.Struct"><code class="xref py py-class docutils literal notranslate"><span class="pre">Struct</span></code></a> instance.</p>
</div>
<p>Compiled Struct objects support the following methods and attributes:</p>
<dl class="method">
<dt id="struct.Struct.pack">
<code class="descname">pack</code><span class="sig-paren">(</span><em>v1</em>, <em>v2</em>, <em>...</em><span class="sig-paren">)</span><a class="headerlink" href="#struct.Struct.pack" title="Permalink to this definition">¶</a></dt>
<dd><p>Identical to the <a class="reference internal" href="#struct.pack" title="struct.pack"><code class="xref py py-func docutils literal notranslate"><span class="pre">pack()</span></code></a> function, using the compiled format.
(<code class="docutils literal notranslate"><span class="pre">len(result)</span></code> will equal <a class="reference internal" href="#struct.Struct.size" title="struct.Struct.size"><code class="xref py py-attr docutils literal notranslate"><span class="pre">size</span></code></a>.)</p>
</dd></dl>

<dl class="method">
<dt id="struct.Struct.pack_into">
<code class="descname">pack_into</code><span class="sig-paren">(</span><em>buffer</em>, <em>offset</em>, <em>v1</em>, <em>v2</em>, <em>...</em><span class="sig-paren">)</span><a class="headerlink" href="#struct.Struct.pack_into" title="Permalink to this definition">¶</a></dt>
<dd><p>Identical to the <a class="reference internal" href="#struct.pack_into" title="struct.pack_into"><code class="xref py py-func docutils literal notranslate"><span class="pre">pack_into()</span></code></a> function, using the compiled format.</p>
</dd></dl>

<dl class="method">
<dt id="struct.Struct.unpack">
<code class="descname">unpack</code><span class="sig-paren">(</span><em>buffer</em><span class="sig-paren">)</span><a class="headerlink" href="#struct.Struct.unpack" title="Permalink to this definition">¶</a></dt>
<dd><p>Identical to the <a class="reference internal" href="#struct.unpack" title="struct.unpack"><code class="xref py py-func docutils literal notranslate"><span class="pre">unpack()</span></code></a> function, using the compiled format.
The buffer’s size in bytes must equal <a class="reference internal" href="#struct.Struct.size" title="struct.Struct.size"><code class="xref py py-attr docutils literal notranslate"><span class="pre">size</span></code></a>.</p>
</dd></dl>

<dl class="method">
<dt id="struct.Struct.unpack_from">
<code class="descname">unpack_from</code><span class="sig-paren">(</span><em>buffer</em>, <em>offset=0</em><span class="sig-paren">)</span><a class="headerlink" href="#struct.Struct.unpack_from" title="Permalink to this definition">¶</a></dt>
<dd><p>Identical to the <a class="reference internal" href="#struct.unpack_from" title="struct.unpack_from"><code class="xref py py-func docutils literal notranslate"><span class="pre">unpack_from()</span></code></a> function, using the compiled format.
The buffer’s size in bytes, minus <em>offset</em>, must be at least
<a class="reference internal" href="#struct.Struct.size" title="struct.Struct.size"><code class="xref py py-attr docutils literal notranslate"><span class="pre">size</span></code></a>.</p>
</dd></dl>

<dl class="method">
<dt id="struct.Struct.iter_unpack">
<code class="descname">iter_unpack</code><span class="sig-paren">(</span><em>buffer</em><span class="sig-paren">)</span><a class="headerlink" href="#struct.Struct.iter_unpack" title="Permalink to this definition">¶</a></dt>
<dd><p>Identical to the <a class="reference internal" href="#struct.iter_unpack" title="struct.iter_unpack"><code class="xref py py-func docutils literal notranslate"><span class="pre">iter_unpack()</span></code></a> function, using the compiled format.
The buffer’s size in bytes must be a multiple of <a class="reference internal" href="#struct.Struct.size" title="struct.Struct.size"><code class="xref py py-attr docutils literal notranslate"><span class="pre">size</span></code></a>.</p>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.4.</span></p>
</div>
</dd></dl>

<dl class="attribute">
<dt id="struct.Struct.format">
<code class="descname">format</code><a class="headerlink" href="#struct.Struct.format" title="Permalink to this definition">¶</a></dt>
<dd><p>The format string used to construct this Struct object.</p>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.7: </span>The format string type is now <a class="reference internal" href="stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate"><span class="pre">str</span></code></a> instead of <a class="reference internal" href="stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a>.</p>
</div>
</dd></dl>

<dl class="attribute">
<dt id="struct.Struct.size">
<code class="descname">size</code><a class="headerlink" href="#struct.Struct.size" title="Permalink to this definition">¶</a></dt>
<dd><p>The calculated size of the struct (and hence of the bytes object produced
by the <a class="reference internal" href="#struct.pack" title="struct.pack"><code class="xref py py-meth docutils literal notranslate"><span class="pre">pack()</span></code></a> method) corresponding to <a class="reference internal" href="functions.html#format" title="format"><code class="xref py py-attr docutils literal notranslate"><span class="pre">format</span></code></a>.</p>
</dd></dl>

</dd></dl>

</div>
</div>


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  <h3><a href="../contents.html">Table of Contents</a></h3>
  <ul>
<li><a class="reference internal" href="#"><code class="docutils literal notranslate"><span class="pre">struct</span></code> — Interpret bytes as packed binary data</a><ul>
<li><a class="reference internal" href="#functions-and-exceptions">Functions and Exceptions</a></li>
<li><a class="reference internal" href="#format-strings">Format Strings</a><ul>
<li><a class="reference internal" href="#byte-order-size-and-alignment">Byte Order, Size, and Alignment</a></li>
<li><a class="reference internal" href="#format-characters">Format Characters</a></li>
<li><a class="reference internal" href="#examples">Examples</a></li>
</ul>
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